Methods and arrangements for melting material which melts only with difficulty



Apnl 1, 1969 J. w. DE RUITER ETAL 3,436,465 METHODS AND ARRANGEMENTS FORMELTING MATERIAL WHICH MELTS ONLY WITH DIFFIGULTY Filed Nov. 17, 1965'INVENTOR. JACOB was RUlTE-R ANTON wmemsrem BY AGENT United StatesPatent C) US. Cl. 131 9 Claims It is known to heat materials meltingonly with difficulty in electric furnaces in which an arc discharge isoperative between two electrodes or a gas discharge is maintained by ahigh-frequency induction field, while the required starting substance isprocessed in pulverulent form. Metal powder in compressed and sinteredform is used as a fusing electrode of an electric arc discharge, but thepulverulent starting substance is likewise supplied to the discharge arein pulverulent state, in which event it falls down through the flame.The melted material is collected on the lower side in a cooled crucible.

The latter method has the advantage that it is not necessary to startfirst with the manufacture of compressed and sintered electrodes. It mayalso be used for non-metallic materials. On the other hand it can beassessed that when the pulverulent material is supplied to the dischargearc, the conversion into melted material is not complete, since theabsorption of a quantity of heat sufficient for melting is dependentupon the temperature and the particle size. The temperatures in'thedischarge are are different from place to place and decrease especiallyfrom the centre towards the outer side so that only part of thedischarge volume has the correct temperature at which material particlesof a given size melt. Furthermore, it is very time-consuming topulverize material to particles of substantially equal size. In order toobtain optimum results, attempts can be made to limit the supply ofpulverulent material to a given region within the discharge volume, butdue to whirling gas flows the power will be scattered and partlyexcessively heated and be burned or it will be insufiiciently heated andit will be collected as such together with the melted material in thecrucible.

The invention has for its object to avoid this disadvan tage and relatesto a method and an arrangement for melting pulverulent material whichmelts only with difiiculty by means of an electricgas discharge inside asheath of high melting-point material, for example, of quartz glass, thegas discharge being operative in the induction field of a high-frequencycoil which surrounds the sheath and is connected to a high-frequencygenerator. According to the invention, the pulverulent material is fedto the working space while the sheath is rotating, as a result of whichthe powder spreads along the circumference of the sheath and constitutesa wall layer and said layer is heated on the inner side to the meltingtemperature of the material by the heat developed in the gas discharge.

The method in accordance with the invention may be used for meltingpulverulent material and conducting away the melted material. In aspecific embodiment for this purpose of an arrangement for carrying outthe method in accordance with the invention, the rotatable sheath isarranged vertically and provided with a transverse wall having a smallaperture and constituting the lowermost boundary of the gas dischargevolume. The pulverulent material supplied is collected immediately abovethis transverse wall. On the wall of the sheath, which may be cooled onthe outer side, the material remains in the pulverulent state, butbetween the pulverulent material and the melted Patented Apr. 1, 1969material a material is formed which is sintered. Contamination of themelted material by particles of material which have not melted is thusavoided. A continuous sup ply of powder may take place on the upper sideof the working space so that the melted material flows away continuously through the aperture in the transverse wall.

In case of melting substances which are electrically conducting or whichare electrically conducting when heated to a high temperature, it mustbe considered that the formation of an electrically conducting pathwithin the field of the high-frequency coil, for example, owing to thefact that the material is sintered, results in that the induction fieldof the high-frequency is screened for a major part so that aninsufficient or at least smaller quantity of highfrequency energy isavailable for the gas discharge. On the inner side of the sheath Wall,provision may be made of receding ribs or thickened parts which preventthe formation of an uninterrupted coating of pulverulent material. Inorder to prevent these receding thickened parts from melting, they aremade of high melting-point material and moreover a satisfactory drainageof heat may be ensured in that channels are provided on the inner sidefor a circulating coolant, for example, gas or liquid. In order tomaintain the dynamic equilibrium, at least two thickened parts should beprovided diametrically,

The diameter of the aperture in the transverse wall is dependent uponthe material to be melted, in which case surface stress, specific weightand similar material properties must be taken into account.

The arrangement provides the possibility of manufacturing wire or thinrods of high melting-point metals, oxides or carbides. For themanufacture of tubes, provi sion is made in the transverse wall at theCentre of the aperture of a mandril of refractory material which isconnected at a plurality of points by narrow radial strips with thetransverse wall. The mandril preferably consists of a material ofsatisfactory thermal conductivity and having a high melting point, sothat the temperature is substantially the same at all places as a resultof a satisfactory heat distribution. As a matter of course, the radialstrips must have a sutficient strength to resist the pressure ofmaterials having a comparatively low flow capacity. This pressure may beconsiderable if the flow of the material must be forced and a highpressure of, for example, a few atmospheres, is maintained in thedischarge space. It is also possible to exhaust the space beneath thetransverse wall to a low pressure. Since the liquid mass of materialfills the aperture, a pressure difference may be maintained without asubstantial loss of material being involved.

A particular advantage exists if the gas in the discharge space has alow pressure. In this case, the material to be melted may be degasedsimultaneously, as a result of Which the formation of gas bubbles in themelted material is counteracted. With the use of the high-frequency gasdischarge satisfactory results can be obtained in the pressure range of1-1O torr, although the flows of whirling gas which are associated withthe rotation of the sheath and which maintain the stabilization of thedischarge have become less important due to the low pressure. In thispressure range, the lengths of the free paths of the electricallycharged gas particles are sufficient for the use of magneticstabilization with the aid of a comparatively low magnetic field ofconstant strength the lines of force of which are parallel to the axisof the sheath. Under the influence of this field, the charged particlesmay cover circular paths and particles having outwardly directed speedsare conducted back into the discharge volume.

The method in accordance with the invention is of particular importancefor the manufacture of granular material. When the sheath rotates at asufficient speed, the coherence of the material flowing out of theaperture in the transverse wall is broken up, as a result of which thematerial spreads from the aperture in separate drops the size of whichis dependent upon the flow capacity and the surface stress of thematerial and upon the speed of rotation of the sheath. At a constantspeed of rotation, grains of material of equal size will be obtainedwhich have a substantially spherical shape.

Alternatively, the method in accordance with the invention may be usedfor manufacturing high melting-point alloys. When the startingsubstances required for an alloy are supplied separately in pulverulentstate to the space situated inside the sheath and above the workingspace, these substances are mixed as a result of the whirling caused bythe rotation. The powder particles of the more readily meltable startingsubstance will melt more rapidly and wet the grains melting only withditliculty, whereupon the melting of these grains is continued and themelted alloy is formed.

High melting-point materials for the manufacture of fuel elements ofreactors may be manufactured in the same manner. It is known to useuranium carbide or plutonium carbide as mixed carbides by adding othercarbides such as zirconium carbide, niobium carbide, tantalum carbideand thorium carbide, in which event the desired composition can beobtained by melting in the high-frequency gas discharge instead ofcarrying out the time-consuming sintering process at high temperature.

The materials obtained in successive melting phases have a high densitywhile the starting materials are distributed over the composed matrix ina uniform manner.

An embodiment of an arrangement for carrying out the method inaccordance with the invention will now be described with reference tothe drawing.

Such an arrangement consists of a rotating gas discharge burnercomprising the tubular sheath 1 which is made of high-melting pointmaterial, for example, of quartz glass. The sheath 1 is provided at bothends with ball-bearings 2 and 3 which are arranged in flat plates 4 and5. These plates are connected to each other by rods 6 and 7. The sheathis vertically arranged and covered on the upper side with a hood 8secured to the fiat plate 4. Two tubes 9 and 10 for supplying one ormore gases suitable for producing an electric discharge are provided inthe hood. Together with an electrically insulatecl through-connection 11consisting, for example, of a vitreous enamel, two current conductors 12and 13 are passed through the hood 8 into the sheath, the ends of theseconductors located inside the sheath being spaced apart by a shortdistance and constituting electrodes 14 and 15 between which an arcdischarge can be produced by the application of an electric voltagethereto. With the aid of such an auxiliary discharge, the gas dischargeinside the sheath 1 can be ignited. For this purpose, it is requiredthat the high-frequency coil 16 which surrounds the sheath 1 and whichis disposed in the current circuit of a high-frequency generator, forexample, a magnetron oscillator, produces a magnetic induction field inthe proximity of the auxiliary discharge. The position of thehigh-frequency coil 16 in which the gas discharge is ignited isindicated with dotted lines. It is therefore rendered possible torelatively displace the coil and the sheath, in the embodiment thesheath 1 being displaceable along the supports 17 and 18.

Approximately at the center of the sheath 1, provision is made of atransverse wall 19 comprising an aperture 20.

A crucible 21 is mounted below this transverse wall and has a rod-shapedsupport 22 by which the crucible 21 bears on a transverse plate 23. Thisplate is secured by means of a ring 24 to the lower flat plate 5. Thering is provided with an aperture 25 which serves for drawing ofi gasand which may communicate with a vacuum pump for reducing the pressurein the lower portion of the sheath 1. The ball-bearings 2 and 3 areclosed by means of locking rings 26 and 27 of a resilient material. Thecrucible 21 may be artificially cooled by a cooling 4 liquid for whichan inlet and an outlet 28 and 29 are indicated on the lower side of thetransverse plate 23.

The inner ring of the lower ball-bearing 3- has secured to it a rotarydisc 30 which is provided with a groove for a rope establishing thetransmission required for the rotation of the sheath 1 with a drivinggear.

After the ignition of the gas discharge the discharge is limited by theadjustment of the speed of rotation of the sheath and of the intensityof the gas supply to a volume inside the sheath 1 of a cross-sectionsuch that the wall of the sheath is not damaged by the developed heat.When the sheath 1 is displaced upwards along the supports 17 and 18, thetransverse wall 19 approaches the gas discharge. At the same time, thestarting substance to be melted is introduced into the working spacethrough a tube 32 which projects outwards through the hood 8 and whichis provided with a filling funnel 33. Under the influence of therotation, the powder is scattered along the wall of the sheath 1 and isat the same time moved downwards so that an accumulation 34 is formed inthe proximity of the transverse wall 19. When the flow of gas isgradually reduced until the supply of gas is stopped completely orsubstantially completely, the gas discharge volume may be adjusted bythe control of the speed of rotation of the sheath so that the powderparticles constituting the inner surface of the collected pulverulentmass melt. A kind of crucible is then formed in the sheath whichprotects the sheath from excessive heating. The heat penetrating intothe powder brings about si-ntering of the powder under the melting layerand consequently constitutes a sufiicient thermal resistance. The sheathmay be cooled on the outer side. No difiiculties are involved inadjusting the supply of the pulverulent starting substance so that anequal quantity of material is conducted away by melting, which materialis collected in the crucible 21.

What is claimed is:

1. A method of melting materials which melt only at relatively hightemperatures comprising supplying gases suitable for producing anelectric discharge in a working space surrounded by a sheath of highmelting point material, maintaining said gas discharge in said sheath bythe induction field of a high frequency coil, rotating said sheath whilesaid melting materials are supplied to said working space in apulverulent state, the speed or rotation being such that the materialpowder spreads along the inner side of said sheath to form a wall layer,and heating said wall layer to its melting temperature by the heatdeveloped by said gas discharge.

2. A method of melting materials which melt only at relatively hightemperatures as claimed in claim 1 further conducting away the meltedmaterial through an aperture in a transverse wall provided in saidsheath.

3. A method of melting materials which melt only at relatively hightemperatures as claimed in claim 2 wherein the speed of rotation of saidsheath is such that the material passing through said aperture isscattered in the form of drops.

4. A method of melting materials which melt only at relatively hightemperatures as claimed in claim 2 wherein at the outlet side of saidtransverse wall the pressure is lower than in the working space of saidsheath.

5. A method of melting materials which melt only at relatively hightemperatures as claimed in claim 4 wherein the gas pressure in theworking space is 110 torr.

6. A method of melting materials which melt only at relatively hightemperatures as claimed in claim 1 wherein at least two powders ofditferent starting substances are simultaneously applied to the workingspace of said sheath.

7. An apparatus for melting materials which melt only at relatively hightemperatures comprising an electric gas discharge burner which isprovided with a sheath of high melting point material, means forrotating said sheath about its axis, a high frequency coil surroundingsaid sheath, and second means for displacing said sheath along 5 thelongitudinal axis thereof relative to said high frequency coil.

8. An apparatus for melting materials which melt only at relatively hightemperatures as claimed in claim 7 wherein said sheath is provided withat least two partitions of thickened parts extending radially from theinner cir- 5 cumference of said sheath.

9. An apparatus for melting materials which melt only at relatively hightemperatures as claimed in claim 8 wherein said thickened parts arehollow tubes of high melting point, non-conducting material, said tubesbeing cooled by a coolant conducted through said tubes.

6 References Cited UNITED STATES PATENTS 3/1964 Van Run l31 5/1966Hikido 13-1 X US. Cl. X.-R. 75-.5

1. A METHOD OF MELTING MATERIALS WHICH MELT ONLY AT RELATIVELY HIGHTEMPERATURES COMPRISING SUPPLYING GASES SUITABLE FOR PRODUCING ANELECTRIC DISCHARGE IN A WORKING SPACE SURROUNDED BY A SHEATH OF HIGHMELTING POINT MATERIAL, MAINTAINING SAID GAS DISCHARGE IN SAID SHEATH BYTHE INDUCTION FIELD OF A HIGH FREQUENCY COIL, ROTATING SAID SHEATH WHILESAID MELTING MATERIALS ARE SUPPLIED TO SAID WORKING SPACE IN APULVERULENT STATE, THE SPEED OR ROTATION BEING SUCH THAT THE MATERIALPOWDER SPREADS ALONG THE INNER SIDE OF SAID SHEAT TO FORM A WALL LAYER,AND HEATING SAID WALL LAYER TO ITS MELTING TEMPERATURE BY THE HEATDEVELOPED BY SAID GAS DISCHARGE.